1. Field of the Invention
The present invention relates to a material for lead frames which can be used as lead frames for LSI and the like.
2. Related Art Statement
Usually, a lead frame has a general shape as depicted in FIG. 1. The lead frame is provided with a silicon chip attached on the central part 1 of the lead frame, and electrodes on the chip are connected to the leading part 2 of the lead frame through minute wires of Au or Al to accomplish the so-called "LSI". The LSI is mounted on a printed board for practical use. Such lead frame is produced usually from a thin plate material having a thickness of about 0.25 mm by punching it out from the material. Hence, a material for lead frames is required to have many characteristic properties such as sufficient mechanical strength to prevent breakage or bending of the thin legs 3 of the lead frame, high electrical conductivity, high thermal conductivity to rapidly dissipate heat generated in LSI chips, and good workability. A conventional material of for lead frames is the so-called 42 alloy having a composition consisting of 42% of Ni and the remainder being Fe, which has a tensile strength of 55-65 kgf/mm.sub.2 as shown in FIG. 2, which, however, has the drawbacks of having a low electrical conductivity of 4% IACS, and insufficient heat dissipation resulting in thermal destruction when it is used as a lead frame for a highly integrated LSI, such as a 256 kb LSI.
Therefore, in order to improve electrical conductivity of a material to be used for a lead frame, there have been developed various types of copper alloys, such as A, B, C and the like, as illustrated in FIG. 2. Alloy A has a composition consisting by weight of 0.1% of Cr, 0.2% of Sn, and the remainder being Cu, and having an extremely high electrical conductivity of 90% IACS, but having a very poor tensile strength of about one-half that the 42 alloy. Alloy B has a composition consisting by weight of 2% of Sn, 0.2% of Fe, 0.03% of P, and the remainder being Cu, and having a high tensile strength of 50-60 kgf/mm.sup.2 similar to that of the 42 alloy, but having a considerably low electrical conductivity of 35% IACS. Alloy C has a composition consisting by weight of 2.4% of Fe, 0.12% of Zn, 0.03% of P, and the remainder being Cu, and a fairly good electrical conductivity of 65% IACS, but having a very poor tensile strength of 40-47 kgf/mm.sup.2. Thus, copper alloys show a tendency that when the electrical conductivity is decreased by having a content of an element other than copper, the strength is increased, while when the electrical conductivity is increased because copper is the primary element present, the strength is decreased. At present, a material has not been known which is economical to produce and satisfies both the requirements of superior strength and good electrical conductivity, simultaneously.